Graphically PGR4 has certainly improved, the weather effects both inside and outside the cockpit are pretty spectacular combined with a very high standard of car and bike modelling make PGR4s overall visuals some of the best seen this gen.
The handling of cars is certainly more realistic than PGR3 and the dynamic weather adds the variety 3 lacked as well as adding some rather evil ice patches and puddles in braking zones, this definitely breaks up the monotonous circuit racing that is common in this genre.
The Career mode is well thought out, making you choose a vehicle to use for a series of events, this means some thought must go into choosing a ride if the championship has a combination of Kudos and hot lap/race events. The introduction of bikes again adds more variety and most do have their uses in arcade and career mode as well as making some fairly comical accident photos. The photo mode certainly encourages you to drive in style and capture those magic moments which you can share online as well as voting on photos other drivers have taken.
Overall PGR4 is a great improvement on 3 and just edges out 2 to be my favourite of the series so far.